1. Field of the Invention
This invention relates to a switching power supply having a first main switch, a second main switch and plural magnetic elements connected in series on their primary sides.
This invention also relates to a switching power supply having a first converter and a second converter connected in series.
2. Description of the Prior Art
In a conventional switching power supply, primary sides of two one-transistor-type converters are connected in series and their secondary sides are connected in parallel (for example, see Patent Literature 1). This structure is suitable for increase in output and reduction in breakdown voltage of main switches.
The conventional switching power supply also has a first main switch and a second main switch for dividing a voltage, and a series circuit formed by a capacitor and an auxiliary switch for resetting magnetic fluxes of transformers (for example, see Patent Literature 2 and Patent Literature 3). This structure enables small voltage stress on the main switches and setting of a duty factor in a broad range.
A specific example of this will now be described. FIG. 1 is a structural view showing an example of the conventional switching power supply.
In FIG. 1, an input voltage Vin, a second main switch Q2, an inductor L4, a primary winding N12 of a transformer T2, an inductor L2, a primary winding N11 of a transformer T1, a first main switch Q1, and a resistor Rsen, which is a current detector unit, are connected in series.
More specifically, the first main switch Q1 has its one end (source) connected with a common potential COM from a negative electrode of the input voltage Vin via the resistor Rsen.
The second main switch Q2 has its one end (drain) connected with a positive electrode of the input voltage Vin.
The inductor L2 and the transformer T1 form a first magnetic element (inductor L2 and transformer T1).
One end of the first magnetic element (inductor L2 and transformer T1) is connected with the other end (drain) of the first main switch Q1.
The first magnetic element (inductor L2 and transformer T1) induces a voltage to be an output Vout by turning on/off of the first main switch Q1 and the second main switch Q2.
The inductor L4 and the transformer T2 form a second magnetic element (inductor L4 and transformer T2).
One end of the second magnetic element (inductor L4 and transformer T2) is connected with the other end (source) of the second main switch Q2, and its other end is connected with the other end of the first magnetic element (inductor L2 and transformer T1).
The second magnetic element (inductor L4 and transformer T2) induces a voltage to be an output Vout by turning on/off of the first main switch Q1 and the second main switch Q2.
Therefore, the first magnetic element (inductor L2 and transformer T1) and the second magnetic element (inductor L4 and transformer T2) are connected in series via a potential point Vm as their connection point.
Parallel to the series circuit formed by the first magnetic element (inductor L2 and transformer T1) and the second magnetic element (inductor L4 and transformer T2), a series circuit formed by a capacitor C8 and an auxiliary switch Q4 is connected.
A secondary winding N21 of the transformer T1 is connected with a diode D1 and a diode D2 and is further connected with an inductor L1, a capacitor C1 and a load Load.
A secondary winding N22 of the transformer T2 is connected with a diode D5 and a diode D6 and is further connected with an inductor L3, the capacitor C1 and the load Load.
Therefore, an output of the secondary winding N21 of the transformer T1 in the first magnetic element (inductor L2 and transformer T1) and an output of the secondary winding N22 of the transformer T2 in the second magnetic element (inductor L4 and transformer T2) are connected in parallel.
The first main switch Q1 and the second main switch Q2 are turned on/off in phase. The first main switch Q1 and the auxiliary switch Q4 are turned on/off in a complementary manner, and the second main switch Q2 and the auxiliary switch Q4 are turned on/off in a complementary manner.
The inductor L2 is a leakage inductance component of the transformer T1 or an external inductor. The inductor L4 is a leakage inductor component of the transformer T2 or an external inductor.
The inductor L2 and the inductor L4 have substantially the same electric properties. The transformer T1 and the transformer T2 have substantially the same electric properties.
In this structure, the transformer T1 and the transformer T2 share the load almost evenly. Therefore, this structure is suitable for an application with a large load.
As the thickness of the transformer T1 and the transformer T2 is reduced, the structure is suitable for an application in which thickness of the whole switching power supply is to be reduced.
The first main switch Q1, the second main switch Q2 and the auxiliary switch Q4 form a MOSFET. A parasitic body diode is provided between the drain and source of the MOSFET.
The capacitor C2 is connected parallel to the first main switch Q1. The capacitor C3 is connected parallel to the second main switch Q2. The capacitor C2 is a parasitic capacity of the first main switch Q1 or an external capacitor. The capacitor C3 is a parasitic capacity of the second main switch Q2 or an external capacitor.
The first main switch Q1 and the second main switch Q2 have substantially the same electric properties. The capacitor C2 and the capacitor C3 have substantially the same electric properties.
With this structure, since the first main switch and the second main switch share voltage stress, an element having a low breakdown voltage can be used. The element having a low breakdown voltage has a low ON-state resistance and is made at a low cost.
Moreover, since the first main switch Q1 and the second main switch Q2 are in phase, a control circuit for them is simple and is made at a low cost.
A voltage generated in the resistor Rsen is utilized for current feedback and for control or protection such as restraint of an excess current. To simplify the circuit structure, the resistor Rsen has its one end connected with the common potential COM.
Thus, in general, because of the arrangement of the resistor Rsen, the arrangement of the series circuit formed by the first main switch, the second main switch, the first magnetic element and the second magnetic element becomes asymmetrical.
The operation in this conventional example of FIG. 1 will now be described.
First, an operating state where the first main switch Q1 is on, the second main switch Q2 is on and the auxiliary switch Q4 is off will be described.
In this case, the series circuit formed by the first magnetic element (inductor L2 and transformer T1) and the second magnetic element (inductor L4 and transformer T2) is excited by the input voltage Vin. The diode D1 and the diode D5 are turned on and the diode D2 and the diode D6 are turned off. The inductor L1 and the inductor L3 are excited.
Then, as the first main switch Q1 and the second main switch Q2 are turned off together, the operating state shifts to an operating state which will be described later.
Next, an operating state where the first main switch Q1 is off, the second main switch Q2 is off and the auxiliary switch Q4 is on will be described.
In this case, the series circuit formed by the first magnetic element (inductor L2 and transformer T1) and the second magnetic element (inductor L4 and transformer T2) is reset by a voltage of the capacitor C8. The diode D1 and the diode D5 are turned off and the diode D2 and the diode D6 are turned on. The inductor L1 and the inductor L3 are reset.
Then, as the auxiliary switch Q4 is turned off, the operating state shifts to the above-described operating state.
In this manner, the transformer T1, the inductor L2, the transformer T2 and the inductor L4 repeat excitation and reset without being magnetically saturated.
Both a charging current and a discharging current of the capacitor C8 flow through a circuit formed by the second magnetic element (inductor L4 and transformer T2), the first magnetic element (inductor L2 and transformer T1), the auxiliary switch Q4 and the capacitor C8.
A voltage induced in the secondary winding N21 of the transformer T1 in the first magnetic element (inductor L2 and transformer T1) is rectified at the diode D1 and the diode D2, then smoothed at the inductor L1 and the capacitor C1, and supplies power to the load Load.
Similarly, a voltage induced in the secondary winding N22 of the transformer T2 in the second magnetic element (inductor L4 and transformer T2) is rectified at the diode D5 and the diode D6, then smoothed at the inductor L3 and the capacitor C1, and supplies power to the load Load.
At the capacitor C1, an output voltage Vout is generated on the basis of a common potential GND as a reference.
In the conventional example of FIG. 1, the input voltage Vin is thus converted to the output voltage Vout. If the ratio of on-time to off-time (duty factor) increases, the output voltage Vout rises. If the ratio of on-time to off-time (duty factor) decreases, the output voltage Vout falls.
In the conventional example of FIG. 1, since the auxiliary switch Q4 and the capacitor C8 suitably reset the first magnetic element (inductor L2 and transformer T1) and the second magnetic element (inductor L4 and transformer T2), the duty factors of the first main switch Q1 and the second main switch Q2 can be set in a broad range.
Moreover, the inductor L2, the inductor L4, the capacitor C2 and the capacitor C3 act to restrain the loss when switching the first main switch Q1, the second main switch Q2 and the auxiliary switch Q4. Particularly, they restrain the loss when turning on the switches.
The characteristics of the conventional example of FIG. 1 will now be described with reference to FIG. 2. FIG. 2 shows operating waveforms of a voltage (Vds_Q1) between the drain and source of the first main switch Q1 and a voltage (Vds_Q2) between the drain and source of the second main switch Q2 in the conventional example of FIG. 1.
As seen from FIG. 2, the stress of the voltage (Vds_Q1) and the stress of the voltage (Vds_Q2) are not balanced with each other. This is affected by the transformer T1, the inductor L2, the transformer T2, the inductor L4 and other parasitic elements, or variations in the on/off timing.
Particularly, the conventional example of FIG. 1 has a characteristic that the symmetry is significantly broken by even a very small resistance Rsen and therefore the voltage (Vds_Q1) and the voltage (Vds_Q2) largely vary from each other.
If the balance between the stress of the voltage (Vds_Q1) and the stress of the voltage (Vds_Q2) is broken, problems arise such as lower efficiency and lower reliability.
Another conventional example will now be described. FIG. 3 is a structural view showing an example of a second conventional switching power supply. The same elements as those of the conventional example shown FIG. 1 are denoted by the same symbols and numerals and will not be described further in detail.
In the conventional example of FIG. 3, with respect to a potential point Vm, which is a connection point between a first magnetic element (inductor L2 and transformer T1) and a second magnetic element (inductor L4 and transformer T2), a bulk capacitor C6 is arranged between the potential point Vm and a common potential COM, which is from a negative electrode of an input voltage Vin, and a bulk capacitor C7 is arranged between the potential point Vm and a positive electrode of the input voltage Vin.
The schematic operation in this conventional example of FIG. 3 converts the input voltage Vin to an output voltage Vout, substantially similarly to the operation in the conventional example of FIG. 1.
The bulk capacitor C6 and the bulk capacitor C7 smooth the potential point Vm. Therefore, these bulk capacitors act to balance a voltage (Vds_Q1) between the drain and source of a first main switch Q1 and a voltage (Vds_Q2) between the drain and source of a second main switch Q2.
The characteristics of the conventional example of FIG. 3 will now be described with reference to FIG. 4. FIG. 4 shows operating waveforms of the voltage (Vds_Q1) between the drain and source of the first main switch Q1 and the voltage (Vds_Q2) between the drain and source of the second main switch Q2 in the conventional example of FIG. 3.
As seen from FIG. 4, the voltage (Vds_Q1) and the voltage (Vds_Q2) are balanced as a whole, but ringing is superimposed thereon.
This is because a ringing path is generated by the addition of the bulk capacitor C6 and the bulk capacitor C7.
As ringing is superimposed on the voltage (Vds_Q1) and the voltage (Vds_Q2), not only the loss of the first main switch Q1 and the second main switch Q2 increases but also the reliability of the entire switching power supply is lowered.
Moreover, the noise characteristic of the switching power supply deteriorates. If the capacitance of the bulk capacitor C6 and the bulk capacitor C7 is increased excessively, the switching power supply is increased in size.
In this manner, there is a tradeoff in reducing the loss, reducing the noise, or restraining the stress on the elements such as the transformer T1, the transformer T2, the first main switch Q1 and the second main switch Q2. It is difficult to realize all these features.
Still another conventional example will now be described. FIG. 5 is a structural view showing an example of a third conventional switching power supply. The same elements as those of the conventional example shown in FIG. 3 are denoted by the same symbols and numerals and will not be described further in detail.
In the conventional example of FIG. 5, primary sides of two converters are connected in series and their secondary sides are connected in parallel.
Specifically, a first magnetic element (inductor L2 and transformer T1), a first main switch Q1, a capacitor C2, a resistor Rsen, a first auxiliary switch Q5, a first capacitor C5, a diode D1, a diode D2, an inductor L1 and a capacitor C1 form a first converter.
More specifically, the first converter has the first magnetic element (inductor L2 and transformer T1) which is connected in series with the first main switch Q1 and induces a voltage to be output by turning on/off of the first main switch Q1.
A second magnetic element (inductor L4 and transformer T2), a second main switch Q2, a capacitor C3, a second auxiliary switch Q6, a second capacitor C4, a diode D5, a diode D6, an inductor L3 and the capacitor C1 form a second converter.
More specifically, the second converter has the second magnetic element (inductor L4 and transformer T2) which is connected in series with the second main switch Q2 and induces a voltage to be an output by turning on/off of the second main switch Q2.
An input voltage Vin, the first converter and the second converter are connected in series.
A bulk capacitor C6 and the first converter, and a bulk capacitor C7 and the second converter are connected in series at a potential point Vm as their connection point on the primary side of the switching power supply.
An output from the transformer T1 and output from the transformer T2 are connected in parallel on the secondary side of the switching power supply. The capacitor C1 is shared by the first converter and the second converter.
Moreover, the first converter and the second converter have substantially the same electrical properties.
Therefore, the first converter and the second converter share the load almost evenly.
The first converter is turned on/off in a complementary manner with the first main switch Q1 and the first auxiliary switch Q5 on the basis of feedback from an output voltage Vout.
The second converter is turned on/off in a complementary manner with the second main switch Q2 and the second auxiliary switch Q6 on the basis of feedback from the output voltage Vout.
The schematic operation of the first converter in this conventional example of FIG. 5 converts the input voltage Vin to the output voltage Vout, substantially similarly to the operation in the conventional example of FIG. 1.
Also the schematic operation of the second converter in the conventional example of FIG. 5 converts the input voltage Vin to the output voltage Vout, substantially similarly to the operation in the conventional example of FIG. 1.
The bulk capacitor C6 and the bulk capacitor C7 act to smooth the potential point Vm and stabilize the potential point Vm.
In the conventional example of FIG. 5, since the first converter and the second converter are independent of each other as described above, there is an advantage that these converters can be easily designed.
However, in the conventional example of FIG. 5, the four active switches, that is, the first main switch Q1, the second main switch Q2, the first auxiliary switch Q5 and the second auxiliary switch Q6, are necessary. Therefore, the structure is complicated, expensive, and hard to miniaturize.
Patent Literature 1: Specification of U.S. Pat. No. 4,685,039
Patent Literature 2: JP-UM-A-6-5390
Patent Literature 3: Japanese Utility Model Registration No.2,591,056
Therefore, such a conventional switching power supply has a problem that the voltage stress on the first main switch Q1 and the voltage stress on the second main switch Q2 largely vary depending on the parasitic elements, the resistor Rsen and the like in the minute circuit.
Moreover, there is a tradeoff in reducing the loss, reducing the noise, or restraining the stress on the elements such as the transformer T1, the transformer T2, the first main switch Q1 and the second main switch Q2. It is difficult to realize all these features.
There is also a problem that the structure of the switching power supply is complicated and hard to miniaturize.